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arxiv: 2604.27963 · v1 · submitted 2026-04-30 · ⚛️ physics.atom-ph · physics.optics

Recognition: unknown

International Optical Clock Comparison Using the European Optical Fiber Network

Marco Pizzocaro , Clara Zyskind , Anne Amy-Klein , Erik Benkler , Sebastien Bize , Davide Calonico , Etienne Cantin , Christian Chardonnet
show 36 more authors
Cecilia Clivati Stefano Condio E. Anne Curtis Simone Donadello S\"oren D\"orscher Chen-Hao Feng Melina Filzinger Jacques-Olivier Gaudron Rachel M. Godun Irene Goti Ian R. Hill Wei Huang Nils Huntemann Matthew Johnson Joshua Klose Jochen Kronj\"ager Alexander Kuhl Rodolphe Le Targat Filippo Levi Burghard Lipphardt Christian Lisdat Jerome Lodewyck Olivier Lopez Helen S. Margolis Maxime Mazouth-Laurol Alberto Mura Benjamin Pointard Paul-Eric Pottie Matias Risaro Billy I. Robertson Marco Schioppo Kilian Stahl Martin Steinel Alexandra Tofful Mads T{\o}nnes Jacob Tunes
Authors on Pith no claims yet

Pith reviewed 2026-05-07 05:42 UTC · model grok-4.3

classification ⚛️ physics.atom-ph physics.optics
keywords optical clocksfiber linkclock comparisonfrequency metrologyytterbium ionmercury clockSI secondinternational metrology
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The pith

Four European labs used an optical fiber network to compare seven optical clocks, with two ytterbium clocks agreeing to within 7.7×10^{-18}.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper reports a two-month campaign in which seven optical clocks at four national metrology institutes were linked through the established European optical fiber network. It shows that independent clocks can be verified against each other at the low 10^{-18} level, as demonstrated by the agreement of the two ^{171}Yb^+ (E3) clocks at NPL and PTB. This level of consistency matters because it supplies direct evidence needed to re-define the SI second on an optical transition rather than the current cesium microwave standard. The campaign also delivered new frequency ratios involving the ^{199}Hg clock with uncertainties improved to the 10^{-17} range.

Core claim

The ^{171}Yb^+ (E3) clocks at NPL and PTB demonstrated agreement within an uncertainty of 7.7×10^{-18}, marking the first international verification of two independently developed optical clocks below one part in 10^{17}. The campaign connected clocks at INRIM, LNE-OP, NPL, and PTB via the European optical fiber network over two months and produced optical frequency ratios with uncertainties ranging from 7.7×10^{-18} to 6.1×10^{-17}.

What carries the argument

The European optical fiber network that transmits stable optical frequency signals between national metrology institutes, enabling remote comparison of clock frequencies without physical transport of the devices.

If this is right

  • The measured ratios supply concrete data for the evaluation of candidate transitions in the redefinition of the SI second.
  • Fiber links remove the need to transport clocks, making regular international comparisons practical.
  • Improved ratios for the ^{199}Hg clock tighten constraints on possible variations of fundamental constants.
  • Clock networks enable differential measurements of gravitational potential for geodesy applications.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If such networks are extended across continents, persistent discrepancies could signal either new physics or underestimated systematics.
  • Continuous operation of the links could support real-time monitoring of clock stability for navigation or earth-science applications.
  • The same infrastructure could be used to test whether the same optical transitions remain identical when viewed from different gravitational potentials.

Load-bearing premise

All systematic effects in the individual clocks and in the fiber links have been correctly identified and quantified so that the reported uncertainties fully capture the true error budget.

What would settle it

A later independent comparison of the same NPL and PTB ^{171}Yb^+ clocks that finds a frequency difference larger than the combined 7.7×10^{-18} uncertainty.

Figures

Figures reproduced from arXiv: 2604.27963 by Alberto Mura, Alexander Kuhl, Alexandra Tofful, Anne Amy-Klein, Benjamin Pointard, Billy I. Robertson, Burghard Lipphardt, Cecilia Clivati, Chen-Hao Feng, Christian Chardonnet, Christian Lisdat, Clara Zyskind, Davide Calonico, E. Anne Curtis, Erik Benkler, Etienne Cantin, Filippo Levi, Helen S. Margolis, Ian R. Hill, Irene Goti, Jacob Tunes, Jacques-Olivier Gaudron, Jerome Lodewyck, Jochen Kronj\"ager, Joshua Klose, Kilian Stahl, Mads T{\o}nnes, Marco Pizzocaro, Marco Schioppo, Martin Steinel, Matias Risaro, Matthew Johnson, Maxime Mazouth-Laurol, Melina Filzinger, Nils Huntemann, Olivier Lopez, Paul-Eric Pottie, Rachel M. Godun, Rodolphe Le Targat, Sebastien Bize, Simone Donadello, S\"oren D\"orscher, Stefano Condio, Wei Huang.

Figure 1
Figure 1. Figure 1: FIG. 1. Map of the clock comparison, showing the location of the optical clocks and of the optical view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Uptimes of a) clocks and b) fiber links during the comparison campaign. Colored regions view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3 view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Allan deviations (ADEV) for each of the measured frequency ratios in the campaign as view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5. Graphical representation of the correlation coefficients between measurements in Table II. view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6. Comparison of the frequency ratios view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7. Comparison of the frequency ratios involving view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8. Comparison of the frequency ratios involving view at source ↗
read the original abstract

Optical clocks have achieved remarkable estimated fractional frequency uncertainties reaching the $10^{-18}$ level and below, enabling applications in fundamental physics, general relativity, and geodesy. However, the challenge of verifying the international consistency of optical clocks remains critical as efforts intensify toward redefining the SI second based on an optical transition or transitions. We report on a two-month international clock comparison campaign involving seven optical clocks in four national metrology institutes (INRIM, LNE-OP, NPL, and PTB) connected via the optical fiber network established in Europe. The campaign resulted in optical frequency ratios with uncertainties ranging from $7.7\times10^{-18}$ to $6.1\times10^{-17}$. Among the results, the $^{171}$Yb$^+$(E3) clocks at NPL and PTB demonstrated agreement within an uncertainty of $7.7\times10^{-18}$, marking the first international verification of two independently developed optical clocks below one part in $10^{17}$. The operation of the $^{199}$Hg clock at LNE-OP (formerly LNE-SYRTE) resulted in frequency ratios with improved uncertainties with $^{171}$Yb$^+$(E3), $^{171}$Yb, and $^{87}$Sr optical clocks. These results provide input for the redefinition of the second and underscore how fiber-linked clock networks can advance metrology and scientific applications.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript reports results from a two-month international optical clock comparison campaign using the European optical fiber network. Seven optical clocks from four national metrology institutes (INRIM, LNE-OP, NPL, and PTB) were compared, yielding frequency ratios with uncertainties ranging from 7.7×10^{-18} to 6.1×10^{-17}. The standout result is the agreement between the ^{171}Yb^+ (E3) clocks at NPL and PTB within an uncertainty of 7.7×10^{-18}, representing the first international verification of two independently developed optical clocks below 10^{-17}. Improved frequency ratios involving the ^{199}Hg clock at LNE-OP are also presented.

Significance. If the uncertainty budgets are complete and correctly evaluated, this work is significant for the redefinition of the SI second. It provides the first direct international verification of optical clock consistency at the 10^{-18} level, supporting the reliability of optical frequency standards. The fiber-linked network approach is shown to be effective for metrology, fundamental physics, and geodesy applications. The multi-institute collaboration and experimental scale are strengths.

major comments (2)
  1. [§4.2, Table 2] §4.2, Table 2: The combined uncertainty of 7.7×10^{-18} for the NPL-PTB ^{171}Yb^+ (E3) ratio is load-bearing for the central claim, but the breakdown does not explicitly quantify time-varying environmental effects on the fiber link (e.g., dispersion or temperature-induced phase noise) over the full two-month campaign; this leaves open whether the quoted uncertainty fully captures all link systematics.
  2. [§5.1] §5.1: The individual clock systematic evaluations (Stark, Zeeman, BBR shifts) for the two ^{171}Yb^+ (E3) systems are presented separately but lack a direct side-by-side comparison of evaluation methods and residual uncertainties; any unaccounted differential bias would undermine the agreement result.
minor comments (2)
  1. [Abstract] The abstract states 'improved uncertainties' for the ^{199}Hg ratios but does not quantify the improvement relative to prior values; this should be stated explicitly for clarity.
  2. [Figure 3] Figure 3 caption: The error bars on the frequency ratio plot are difficult to discern at the 10^{-18} scale; increasing line thickness or using a log scale inset would improve readability.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We are grateful to the referee for their positive evaluation of our manuscript and for identifying areas where additional clarity can be provided. We have carefully considered the comments and revised the manuscript to address them fully. Our responses to the major comments are as follows.

read point-by-point responses

  1. Referee: [§4.2, Table 2] §4.2, Table 2: The combined uncertainty of 7.7×10^{-18} for the NPL-PTB ^{171}Yb^+ (E3) ratio is load-bearing for the central claim, but the breakdown does not explicitly quantify time-varying environmental effects on the fiber link (e.g., dispersion or temperature-induced phase noise) over the full two-month campaign; this leaves open whether the quoted uncertainty fully captures all link systematics.

    Authors: The fiber link uncertainty is based on the observed stability and repeatability over the entire campaign duration, which inherently includes the effects of time-varying environmental factors like temperature fluctuations and dispersion. We have revised §4.2 to provide a more explicit description of this evaluation process, including how the phase noise measurements account for these variations across the two-month period. revision: yes

  2. Referee: [§5.1] §5.1: The individual clock systematic evaluations (Stark, Zeeman, BBR shifts) for the two ^{171}Yb^+ (E3) systems are presented separately but lack a direct side-by-side comparison of evaluation methods and residual uncertainties; any unaccounted differential bias would undermine the agreement result.

    Authors: We acknowledge that a side-by-side comparison would facilitate easier verification of consistency between the two independent evaluations. In the revised manuscript, we have included a new comparative table in §5.1 that lists the evaluation methods and residual uncertainties for each shift (Stark, Zeeman, BBR) for both the NPL and PTB clocks. This addition demonstrates that the methods are compatible and that any potential differential biases are well within the combined uncertainties, thereby reinforcing the validity of the agreement. revision: yes

Circularity Check

0 steps flagged

No significant circularity: direct experimental frequency ratios from clock comparisons

full rationale

The paper is an experimental measurement campaign reporting observed frequency ratios between optical clocks connected by fiber links, with uncertainties from evaluated systematic budgets. No derivation chain exists in which a claimed prediction or first-principles result reduces by construction to fitted inputs, self-definitions, or load-bearing self-citations. The central result (NPL-PTB Yb+(E3) agreement at 7.7e-18) is a measured difference, not a quantity forced by prior parameters or ansatzes. Self-citations, if present for prior clock characterizations, are not load-bearing for any mathematical reduction here; the result stands as independent data against external benchmarks. Concerns about completeness of uncertainty budgets address potential unaccounted systematics (correctness), not circularity.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

This is an experimental measurement report; no new theoretical entities, free parameters, or ad-hoc axioms are introduced beyond standard assumptions of atomic physics and optical metrology.

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Reference graph

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